Semiconductor devices include input and/or output (I/O) circuits to provide internal signals to an external communication channel and/or to receive signals from the external communication channel. High speed communication systems such as Gunning transceiver logic (GTL), stub series terminated logic (SSTL), and high-speed transceiver logic (HSTL) keep the channel voltage swing small to reduce transmission delay. This reduction in channel voltage swing increases the susceptibility to noise and signal reflection due to termination impedance mismatch. In order to match termination impedances better, some integrated circuits use calibration circuits to match on-chip termination impedances to off-chip precision resistors. However the combination of transistor and resistor elements used to form the on-chip termination impedances in complementary metal-oxide-semiconductor (CMOS) manufacturing processes have some degree of non-linearity, making accurate matching difficult.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.